OCT is a non-invasive optical imaging technique which produces depth-resolved reflectance imaging of samples through the use of a low coherence interferometer system. OCT imaging allows for three-dimensional visualization of structures in a variety of biological systems not easily accessible through other imaging techniques, including but not limited to the retina of the eye.
Vascular visualization and quantitative information of blood flow is very important for the diagnosis and treatment of many diseases. In OCT imaging, a type of phase sensitive analysis called Doppler OCT is the primary form of vascular visualization and diagnostic. Phase is a type of high resolution position measurement of a reflection along the optical path length of the imaging system, which is cyclic of the frequency of half the wavelength of the imaging light. A depth position change of half the imaging wavelength will produce the same phase measurement. Changes in phase are proportional to the axial flow, the flow component parallel to the imaging direction designated by v(cos θ), where v is the velocity of the flow and θ is the angle between the flow direction and the imaging light. Phase noise in the system based on the local signal to noise ratio determines the minimum axial flow measurement, which limits the visualization of flow in cases where v or cos θ is very small. For cases such as in the retina, a lot of flow is nearly perpendicular to the imaging direction such that θ˜90° and cos θ˜0. In these cases, the velocity v of the flow must be extremely high to be able to visualize the flow with this method.
The progress of development in OCT is towards faster imaging technologies and techniques in order to image larger regions in the same amount of time. In order to maintain fast imaging speeds, Doppler OCT imaging techniques only use a few successive depth reflectivity measurements called A-scans (the typical number is around 5), and average the phase change between each of them. The limited statistics and the short time between the A-scans (and also the phase measurements) severely limit the minimum observable axial flow, allowing for only visualization of the fastest flows.
Demonstrated variance calculations of the phase changes for this situation do not add additional motion contrast to the images. This lack of additional contrast is because the phase error due to the local signal to noise ratio dominates the calculations in all regions except in the same fast flow regions visualized with the Doppler OCT technique.
Speckle analysis looks at intensity variations of images, which has limited work demonstrated in the field of OCT. Most of the work with speckle in OCT has been directed towards the reduction of speckle artifacts from multiple reflections within the sample to improve image quality. Demonstrated speckle analysis techniques utilize spatial variations of intensity from a single static image to characterize regions and identify regions of flow. These techniques are only capable of analyzing regions much larger than the spatial resolution of the imaging system and have typically been used in non-OCT imaging situations which do not have the depth discrimination capabilities of OCT.
Accordingly, there is a need in the field of OCT for an accurate and efficient method to ascertain flow of biological fluids to assist in the diagnosis and treatment of many diseases. In particular, there is a need to develop a method capable of estimating transverse flow velocity and to ascertain motion contrast with OCT systems.